Draft control systems of the general type referred to above have long been known in the art, the vast majority of which depend upon some form of hydraulic power lift, or the like, controlled by a valve which is actuated first, in response to a manual setting to select a working depth and second, in response to changes in draft force or load wherein the selected depth is too deep, for example, to enable the tractor-implement outfit to proceed at a desired rate. Such changes normally occur because of increase in soil density, wheel slippage, etc. Also, the vast majority of such systems are based upon the use of a mechanical spring means against which the draft forces are balanced; such spring means already known may be coiled compression springs; bars deflectable in bending, torsion, etc.; leaf springs and the like. At least one other known system employs hydraulic means incorporated in the draft linkage. All of these systems yield essentially the same end result; namely, a signal to the valve means for the hydraulic power lift or equivalent poweradjusting means.
One problem, in any case, is caused by inherent deficiencies in the linkage means for transmitting the signal to the valve, because of such factors as lost-motion, limitations on space, etc., all of which delay response of the valve to the signal.
Assignee's U.S. Pat. No. 2,940,550 to Du Shane, may be taken as representative of a spring-responsive system employing mechanical servo linkage as the means for feeding the draft signal to the valve means. This patent shows the spring means as a bar deflectable in bending. Assignee's U.S. Patent to Du Shane U.S. Pat. No. 2,921,638 discloses the spring means as a coiled compression spring. For general purposes of the present invention, these may be taken as equivalents and as representative of mechanical spring means as distinguished from hydraulic sensing means such as that disclosed in Assignee's U.S. Pat. No. 3,990,520 to Koch. A second characteristic of Du Shane U.S. Pat. No. 2,921,638 is that the valve means may be selectively responsive to draft forces or to the vertical position of the implement (as initially selected manually) or to a combination of both. In this arrangement, in particular, the selectivity as to which response will control the valve means adds further to delaying effects on the transmission of the signal.
The Koch patent solves the problem by interposing force amplifying means in the signal transmitting means but in response to hydraulic sensing of the draft forces. The Koch patent does not consider the multi-functional characteristic of Du Shane U.S. Pat. No. 2,921,638. The force-amplifier provided by the present invention is significantly more important than in a case involving hydraulic sensing (Koch). In a mechanical sensing system, as here involved, the motion available from the load spring is often relatively small and, without amplification, frequently not sufficient to overcome reaction forces in the linkage. Unless and until the reaction forces are overcome, there will be little or no signal or at best a weak signal. Thus, the basic differences are seen to flow from the two types of sensing means, hydraulic vs. mechanical, even though the amplification function and even structure may be similar.